The correct answer is option 2: Stratosphere. The ozone layer depletion is a major phenomenon that primarily occurs in the stratosphere. The stratosphere is the second layer of Earth`s atmosphere, located above the troposphere (option 1). The ozone layer is a specific region within the stratosphere, where ozone (O3) molecules are concentrated.

Ozone depletion is caused by the release of certain chemicals, such as chlorofluorocarbons (CFCs) and halons, into the atmosphere. These chemicals react with and destroy ozone molecules, leading to a thinning of the ozone layer. The depletion of the ozone layer is a serious environmental issue because it allows more ultraviolet (UV) radiation from the Sun to reach the Earth`s surface.

The thermosphere (option 3) and exosphere (option 4) are higher layers of the atmosphere, located above the stratosphere. While these layers are influenced by solar radiation, they are not the primary site of ozone depletion. Therefore, option 2, the stratosphere, is the correct answer.